This invention relates generally to kaolin clays, and more specifically relates to a process for beneficiating a kaolin clay to improve the rheological properties of a high solids suspension subsequently formed from the beneficiated clay, thereby rendering same more suitable for paper and paperboard coating applications. Similarly the rheological properties of the kaolin are so improved by the invention as to render same more readily shippable and handleable as an aqueous suspension or slurry, by virtue of such slurries exhibiting reduced high shear and low shear viscosities.
Kaolin clay coating pigments having very fine particle size and high brightness characteristics, are widely utilized in the coating of merchant grade papers and various types of paperboard wherein high gloss and smoothness of coating is required. Typically, these pigments are applied as a high solids aqueous suspension, i.e., a suspension including from approximately 60-75% by weight of clay solids. The size distribution of prior art pigments used for such purposes are usually such that of the order of 90-100% by weight thereof are of less than 2 microns equivalent spherical diameter (E.S.D.). Typically, further, the brightness characteristics, as measured by the standard specification established by TAPPI procedure T-646m-54, are of the order of at least 90.
Among the further qualities of a high solids coating clay slurry, which are of paramount importance for achieving high quality coatings, are the viscosity characteristics of same. It may be noted in this connection that the term "viscosity" as used herein with respect to clay slurries, refers to such characteristics as determined by the procedures of TAPPI Method T 648 su-72, as revised in 1972. This method sets forth specific procedures for determination of both the "low shear" and "high shear" viscosity. The latter, i.e. the high shear viscosity, is considered of special importance in evaluating a high solids clay slurry for the aforementioned coating purposes.
While numerous products are known and commercially available which are quite adequate (and in many instances of excellent quality) with respect to brightness, and to a lesser extent, particle size distribution, the high shear rheological qualities of these coating clays are less than would be ideally sought for. Efforts have therefore been made over the years to improve the rheological properties of such clays. Reference may be made for example to Turner, U.S. Pat. No. 4,334,985, disclosing a method wherein a 10 to 20% solids aqueous suspension of a kaolin clay is subjected to a selective theological separation, by mixing the suspension with from about 0.001% to 0.1% by weight of dry clay, of a high molecular weight anionic polymer. The treated suspension is then allowed to separate into a sedimented phase, and a supernatant phase which is found to be substantially free of aggregates. The supernatant phase containing the beneficiated kaolin pigment in suspension is then separated from the sedimented phase. The suspended product of the process can then be conventionally processed, e.g., bleached, flocced, filtered and washed, then re-dispersed and dried or mixed with previously prepared dry material, to yield a high solids (e.g. 60% to 75% solids) coating slurry having improved rheological properties.
It has long been known that smectite and other swelling clays are a basic cause of high viscosity characteristics of kaolin slurries. Virtually all kaolin clays contain some form or amount of swelling clay such as smectite, even where such quantities are below XRD detectable levels. Even relatively small concentrations of swelling clays (beyond XRD detectability) found in most kaolin grades, exaggerate the detrimental effect of clay slurry viscosities especially in the presence of higher salt concentrations. While it is therefore clear that separation of the minute smectite component from the kaolin would be beneficial in improving the viscosities of aqueous slurries formed from the beneficiated kaolin, only limited success has thus far been achieved. Mostly such success has been attained by indirection. For example, in the mentioned 4,334,985 patent, it is possible that preferential separation of aggregates may remove some smectite as well since the latter may be preferentially found in such aggregates. Mechanical working of the kaolin can also foster release of the smectite contaminants, and preferential flotation can sometimes be helpful in removing such released material from the kaolin slurry. In general, however, efforts to directly and preferentially separate the smectite contaminants have met with very limited success.
In accordance with the foregoing, it may be regarded as an object of the present invention, to provide a method for beneficiating a kaolin clay to improve the high and low shear viscosities of a high solids suspension subsequently formed from the beneficiated pigment.
It is a further object of the invention, to provide a method for beneficiating a kaolin clay which includes quantities of smectite contaminants, which by directly separating such smectites, reduces the low and high shear viscosities of high solids aqueous slurries of the beneficiated kaolin.